Rule 465.2.c.9
If there is more than one unit in which this situation applies to, each unit is dealt with individually. The assigning player chooses which ability or effect applies, and then resolves the assignment. If this creates a situation where now more than one unit must be assigned with the same priority, those units may be assigned damage in any order as normal within that priority. Example: Two copies of Caitlyn, Patrolling with the Backline ability ("I must be assigned combat damage last.") have been given the Tank ability ("I must be assigned combat damage first."). A player assigning damage to these two Caitlyns and one unit with no abilities could choose to fulfill both Caitlyns’ Tank requirements by assigning them both damage before the other unit.